I. Field of the Invention
The present invention generally relates to methods for preparing fluid catalytic cracking (FCC) catalysts having enhanced acid sites. More specifically, the present invention is directed to aqueous methods for preparing improved FCC catalysts by contacting a catalyst with an aqueous solution including sources of both phosphorus and aluminum having specific characteristics and under specified conditions to enhance the acid sites of the catalyst.
II. Description of the Background
Catalytically controlled processes, including fluid catalytic cracking (FCC), are valuable refining processes employed to upgrade heavy hydrocarbons to higher valued products. In particular, the cracking of hydrocarbon feedstocks to produce hydrocarbons of preferred octane rating which boil in the gasoline range is widely practiced. This cracking uses a variety of solid catalysts typically comprising at least one synthetic crystalline material to give more valuable end products. Cracking is ordinarily employed to produce gasoline as the most valuable product. Cracking is generally conducted at temperatures of about 750-1100.degree. F., preferably about 850-950.degree. F. and at pressures up to about 2000 psig, preferably about atmospheric to about 100 psig. In cracking, the feedstock is usually a petroleum hydrocarbon fraction such as straight run or recycled gas oils or other normally liquid hydrocarbons boiling above the gasoline range.
Over 1100 tons per day of FCC catalyst is used worldwide in over 200 Fluid Catalytic Cracking Units (FCCUs). During the cracking reaction, the catalyst is contaminated by elements deposited from feedstocks. Some contaminants, like the alkali metals, deactivate the catalyst without changing the product distribution. Others, however, including iron, nickel, vanadium and copper, effectively poison the catalyst by altering the selectivity and activity of the cracking reactions if allowed to accumulate on the catalyst. A catalyst, so poisoned with these metals, produces a higher yield of coke and hydrogen at the expense of the more desirable gasolines and butanes. Examples of such poisoning may be found in U.S. Pat. No. 3,147,228 where the yield of desirable butanes, butylenes and gasoline dropped from about 59 to about 49 volume percent as the contamination of the catalyst with nickel and vanadium increased, from 55 ppm to 645 ppm nickel and 145 ppm to 1480 ppm vanadium. Because many cracking units are limited by coke burning or gas handling facilities, increased coke or gas yields require a reduction in conversion or throughput to stay within the unit capacity.
In FCCUs, the bulk of contaminant elements from the feedstock remain in the circulating catalyst system. Through the cycles of cracking, fresh catalyst deactivation is caused by contaminant blockage of active sites by metals, including nickel, vanadium, copper and iron. Deactivation may also occur as the result of steam catalyzed by contaminants such as vanadium and sodium. To compensate for decreased FCC feedstock conversion and product selectivity, a portion of the circulating equilibrium catalyst is regularly withdrawn and replaced by fresh catalyst added to the system. This withdrawn or spent catalyst contaminated with various metals must then be properly disposed of.
Both because of the expense involved in replacing spent catalyst with fresh catalyst and because of the expense involved with the environmentally safe disposal of metal-contaminated catalyst, there have been many efforts to demetallize and reuse the contaminated catalyst. In conventional demetallization processes, portions of the metal contaminants are removed from the spent FCC catalyst by pyrometallurgical methods, e.g., calcining, sulfiding, nitrogen stripping and chlorinating, followed by hydrometallurgical methods, e.g., leaching, washing and drying, to produce a demetallized spent catalyst for reuse in the FCCU. Such a demetallization (DEMET) process is described in U.S. Pat. No. 4,686,197, incorporated herein by reference. The '197 patent describes an improved demetallization process. Also referenced are prior demetallization processes which include the chlorination at elevated temperatures of alumina, silica alumina and silica catalysts contaminated with metals. See, for example, U.S. Pat. Nos. 3,150,104; 3,122,510; 3,219,586; and 3,182,025. Also referenced are demetallization processes which do not primarily involve chlorination of the catalyst. See, for example, U.S. Pat. Nos. 4,101,444; 4,163,709; 4,163,710; and 4,243,550.
Prior demetallization processes, such as the DEMET process described in the '197 patent have most frequently employed calcining and sulfiding steps performed at about 787.degree. C. followed by chlorination at 343.degree. C. The offgases from the reactor are scrubbed, while the removed contaminant metals are precipitated and filtered for disposal in the same manner used for spent catalyst or disposed through any acceptable Best Demonstrated Available Technology (BDAT) method for the recycling of metals.
The recycling of demetallized spent FCC catalyst has reduced the requirements for fresh catalyst additions, reduced the generation of catalyst fines and reduced the disposal problem of spent catalyst. Conventional demetallization processes remove contaminants known to be detrimental to conversion, to product selectivity and to the mechanical performance of the FCC.
While the DEMET process based upon the '197 patent and the prior processes described therein have provided methods for demetallization of spent FCC catalyst, those methods have not been entirely or universally acceptable. Operating conditions are severe and must be strictly maintained. Therefore, the process has found only limited use. For these and other reasons, there has been a long felt but unfulfilled need for a more economical, more efficient, easier and safer method for demetallizing FCC catalysts and for enhancing the acid sites thereon. The present invention solves those needs.